1. Field of the Invention
The invention relates to image forming apparatuses such as a copying machine and a printer which utilizes electrophotography. The invention is directed, in particular, to an image forming apparatus that transfers a toner image formed on an image carrying body onto a recording sheet held on a transfer drum.
2. Description of the Related Art
In image forming apparatuses such as a copying machine and a printer, a laser beam is irradiated onto a photoreceptor, which is an image carrying body, to thereby form a latent electrostatic image, and the thus-formed latent electrostatic image is visualized by depositing toner on the latent electrostatic image to thereby form a toner image. Then, the thus-formed toner image is transferred onto a sheet to thereby produce a printed image. Among these image forming processes is a process in which the toner image formed on the photoreceptor is transferred onto a sheet by a transfer device. One such process involves the steps of: forwarding a sheet to a position confronting the photoreceptor by, e.g., causing the sheet to be electrostatically adsorbed to the transfer drum; and applying a voltage whose polarity is opposite to the polarity of the toner to the sheet from the back side of the sheet, so that the toner image formed on the photoreceptor is be transferred onto the sheet.
In transferring a toner image by adsorbing a sheet to the transfer drum this way, sheet adsorbing performance of the transfer drum may depend on how the sheet is electrically charged. That is, if the force through which the sheet is adsorbed to the transfer drum is weak, the sheet will get separated from the transfer drum and will therefore adhere to the photoreceptor, and this may cause a paper jam (POP jam, or "paper on the photoreceptor jam"). If the paper jam is sensed belatedly in this case, the sheet is continuously forwarded as the photoreceptor rotates, so that its head portion is threaded into a portion confronting the cleaning device by the time the operation of the apparatus is stopped. If the transfer drum is pulled in the axial direction in order to remove the jammed sheet, the head portion of the sheet gets torn as the sheet is taken out of the cleaning device, thus leaving a torn piece of paper inside the image forming apparatus. If the torn piece of paper is left inside the apparatus, it causes such inconvenience as damaging the photoreceptor and the cleaning device upon reactivating the apparatus next time. Hence, a jammed sheet should be removed from the apparatus without leaving a torn piece of paper therein.
To avoid such inconvenience, an apparatus has been proposed in which a jam sensor is disposed downstream of a position where the photoreceptor confronts the transfer drum so as to confront the photoreceptor. With this configuration, a sheet adhering to the photoreceptor is sensed to thereby stop a driving system of the apparatus. In this apparatus, a sheet adhering the photoreceptor is directly sensed with the jam sensor, and the operation of the apparatus is stopped before the head portion of the sheet is threaded into the cleaning device. An optical sensor including a light emitting element such as a LED (light emitting diode) and a photodetecting element such as a PD (photodiode) is frequently used as the jam sensor in view of its advantages of being inexpensive and not being in contact with the photoreceptor.
Further, an image forming apparatus disclosed in Japanese Unexamined Patent Publication No. Hei. 2-156270 employs a device that judges arrival of a sheet through a comparison with a photodetection value of a photoreceptor surface portion by appropriately adjusting either the amount of light emitted by the light emitting element or the photodetection sensitivity of the photodetecting element, when a paper jam occurs.
Such an image forming apparatus, being capable of sensing a paper jam quickly, can prevent a sheet from entering into the portion confronting the cleaning device. Hence, such an image forming apparatus also provides the advantage that a sheet is not torn when the transfer drum is pulled out.
However, the aforementioned image forming apparatus has the following problems.
Where a paper jam is sensed by the aforementioned optical sensor, it is difficult to distinguish untransferred toner on the photoreceptor from a sheet adhering to the photoreceptor. That is, since a sheet is distinguished based on a difference in the intensity of reflected light, it is particularly difficult to distinguish a sheet from untransferred toner on the photoreceptor which has a high reflectance, such as yellow toner used in a full-color image forming apparatus. Further, where a paper jam occurs during formation of an image on the back side of a sheet in a double-sided image forming operation, it is also difficult to distinguish untransferred toner on the photoreceptor from a previously formed image, disabling correct detection of such a paper jam.
When a paper jam is not sensed correctly, the apparatus cannot be stopped quickly, which brings about a problem that the threading of a sheet into the portion confronting the cleaning device with the sheet adhering to the photoreceptor cannot be prevented reliably even if the optical sensor is employed.